1. Field of the Invention
The present invention relates to an endpoint detecting of polishing processing of a semiconductor device, and more particularly to a method of detecting an endpoint in smoothing of a wafer surface and its apparatus, a polishing method with an endpoint detecting function and its apparatus, and a method of manufacturing a semiconductor device using the same.
2. Related Background Art
A semiconductor device is manufactured by forming a film on a silicon wafer (hereinafter simply referred to as wafer) and forming an element or wiring pattern through an exposure in a desired pattern and an etching process of the exposed portion. Subsequently to forming the element or wiring pattern as described above, a transparent layer insulating film made of SiO2 or the like is formed to cover the element or wiring pattern and the next element or wiring pattern is formed on the layer insulating film, thus causing the manufactured semiconductor device to have a laminated structure.
In order to form an element or wiring pattern on a certain layer on a wafer and a layer insulating film so as to cover it and further to form an element or wiring pattern as the next layer on this layer insulating film, an exposure light focusing condition (an exposure condition) must be uniform over the entire film. The under element or wiring pattern, however, generates an uneven surface of the layer insulating film provided to form the next layer on the element or wiring pattern layer on the wafer. Particularly in recent years, a pattern formed on the wafer tends to have a more fine-grained and multi-layered structure so as to achieve a high-precision and high-density semiconductor device, thereby increasing the unevenness on the surface of the layer insulating film to be formed. The increase of the unevenness on the surface of the layer insulating film makes it hard to achieve a uniform exposure condition over the entire film formed on the layer insulating film, and therefore the layer insulating film is smoothed before forming the film.
For this smoothing processing, there is conventionally used a method of realizing a smooth film by polishing a surface by means of chemical and physical effects (CMP: Chemical mechanical polishing). This CMP processing is described below by using FIG. 20.
In this diagram, a pad 1 is provided on a surface of a polishing disk 2 in a polishing machine to be used. The pad 1 is a sheet made of porous hard sponge material having fine holes on its surface. The polishing disk 2 is rotated and slurry 5 which is fluid abrasive including fine abrasive grains is added and applied on a surface of the pad 1. Then, a wafer not shown in a wafer chuck 3 is pressed to the pad 1, thereby causing a layer insulating film on the surface of the wafer to be polished by the pad 1.
It should be noted here that a rotary speed is different between a central portion of the rotating polishing disk 2 and its surrounding portion and therefore the wafer chuck 3 is moved in a radial direction of the polishing disk 2 or rotated so that the entire layer insulating film on the wafer is polished to have a uniform film thickness. This polishing is performed by abrasive grains of the slurry 5 getting into fine holes of the pad 1 to be held therein. If a lot of wafers are polished, however, the pad 1 wears out on its surface, thereby decreasing a polishing performance of the pad 1 or causing a serious condition in which the layer insulating film on the wafer surface has flaws due to contaminants adhering to the surface of the pad 1. Accordingly, a dresser 4 is provided to shave the surface of the pad 1 for a regeneration of the pad surface.
The CMP processing is as set forth in the above. As an important problem in this CMP processing, there is an endpoint detection for terminating polishing when the layer insulating film on the wafer surface has been polished into a predetermined film thickness. The endpoint detection in the CMP processing has been controlled initially by calculating a processing time based on a previously evaluated polishing rate or by detaching the wafer from the CMP processing machine whenever polishing has been performed for a predetermined time and directly measuring a film thickness of the layer insulating film. In these methods, however, the detection cannot be precisely controlled due to uneven polishing rates and further the control takes plenty of time.
To solve these problems, there is disclosed an in-situ measuring system capable of an endpoint detection on an actual wafer by measuring a film thickness of a layer insulating film while polishing it in Japanese Patent Unexamined Publication No. 9-7985.
As shown in FIG. 20, this system is provided with a detection window 6 penetrating the polishing disk 2 and the pad 1, so that the layer insulating film on the wafer surface is irradiated with a laser light having a single wavelength from the detection unit 8 via the detection window 6, the detection unit 8 detects an interference light between a reflected light from the surface of the layer insulating film and a reflected light from a pattern formed under the layer insulating film, and the film thickness evaluation unit 7 detects a variation of a film thickness of the layer insulating film based on a variation P of a detected intensity of the interference light, thereby enabling an endpoint detection of polishing processing.
Referring to FIG. 21, there is shown a detected intensity variation P of the interference light detected by the detection unit 8 in FIG. 20, the detected intensity variation periodically changing as shown in the graph. The maximum amplitude of the interference light in this condition depends upon the layer insulating film formed on the wafer surface and a reflectance of the pattern, a period of the interference light depends upon a wavelength of the emitted laser light, a film thickness of the layer insulating film, and a refractive index of a film material, and an amplitude of the interference light varies with a change of a distance between the surface of the layer insulating film under polishing processing and a pattern surface of the previous layer immediately under the layer insulating film (in other words, a film thickness of the layer insulating film). Therefore, assuming an interference light intensity I at time t, the layer insulating film has a film thickness causing an interference light of the intensity I.
Therefore, a focus can be detected by previously calculating or evaluating in an experiment the interference light intensity I at which the film thickness of the layer insulating film is a predetermined thickness which is an endpoint of the CMP processing (in other words, the entire surface of the layer insulating film is uniformly smoothed), by measuring the interference light intensity with the film thickness evaluation unit 7 during the CMP processing of the wafer as described with referring to FIG. 20, and by determining an endpoint of the CMP processing when the measured intensity becomes equal to the predetermined intensity I.
The interference light intensity varies as indicated by the curve P in FIG. 21 with a progression of polishing the layer insulating film on the wafer surface. This intensity variation P with an elapsed time shows a slow movement. Therefore, a gradient of the curve P is low and therefore even if a predetermined intensity I is detected, it is hard to detect it accurately. Accordingly the conventional in-situ measurement is effective for a relatively large processing amount (polishing amount), while it is often incapable of detecting an endpoint accurately in case of a small processing amount or according to a film structure.